Serial-parallel analog-to-digital converter using voltage subtraction

ABSTRACT

An analog-to-digital converter includes a first comparator which receives an analog input. The first comparator generates a plurality of reference voltages which establish a first continuous range of voltage gaps. The first comparator has a plurality of quantizing outputs, one of which will be at a logical one and the rest of which will be at logical zeros in order to indicate which of the voltage gaps encompasses the analog input voltage. The first comparator also has a reference current output whose magnitude is representative of which voltage gap encompasses the analog input voltage. A first encoder receives the quantizing outputs of the first comparator and generates a binary number which represents which of the voltage gaps the analog input voltage is encompassed by and which constitutes the most significant bit group of the binary digital representation of the analog input voltage. A voltage subtractor circuit receives as inputs the reference current output of the first comparator and the analog input. The voltage subtractor produces an output voltage which represents the voltage difference between a first boundary of the voltage gap which encompasses the analog input voltage and the value of the analog input voltage. The voltage subtractor output voltage is produced concurrently with the action of the first encoder, thereby providing overlapping operations for increased conversion speeds. This voltage is provided as an input to a second comparator which compares the output voltage produced by the voltage subtractor with a plurality of internal reference voltages which form a second continuous range of voltage gaps. The second comparator has a plurality of outputs, one corresponding to each of the voltage gaps encompassed by the reference voltages in the second comparator. The second comparator produces a logical one on an output corresponding to the particular voltage gap which encompasses the difference voltage produced by the voltage subtractor. A second encoder receives as inputs the outputs of the second comparator and generates a binary output which represents the difference voltage produced by the voltage subtractor and which constitutes the least significant bit group of the analog input voltage. In combination, the output of the first encoder and the output of the second encoder form a binary digital representation of the analog input voltage, including the most significant bits and the least significant bits thereof.

RELATED APPLICATIONS

This application is related to the following patent applications of theassignee of the present invention:

Ser. No. 764,499 of Kreinick et al entitled Analog-To-Digital Converter,now abandoned.

2. Ser. No. 764,491 of Kreinick et al entitled Voltage Subtractor ForSerial-Parallel Analog-To-Digital Converter, now U.S. Pat. No.4,124,824.

3. Ser. No. 807,470 of Shaw et al entitled Serial-ParallelAnalog-To-Digital Converter Using Reference Voltage Level Shifting.

4. Ser. No. 807,472 of Shaw et al entitled Reference Voltage LevelShifting Circuit For Serial-Parallel Analog-To-Digital Converter.

BACKGROUND OF THE INVENTION

The present invention relates to analog-to-digital conversion and, inparticular, to a serial-parallel analog-to-digital converter withoverlapping operations.

An analog-to-digital (A/D) converter encodes a voltage or current signalinto a digital representation with respect to a known reference. Itsaccuracy is determined by the number of significant digits of thedigital representation (typically the number of bits in a binaryrepresentation). The bandwidth of an A/D converter is determined by itsconversion speed--the amount of time required to convert an appliedinput quantity to an equivalent digital representation. A widely appliedclass of A/D converters function by comparing an applied analog inputsignal to a known reference to produce a discrete or quantized signaland then encoding the quantized signal to produce a digitalrepresentation of the analog input. The particular A/D converters withinthis class range form what are called "serial" A/D converters incontrast to what are called "parallel" A/D converters. Serial A/Dconverters encode the applied analog input signal one bit at a timestarting with the most significant bit and progressing sequentiallytowards the least significant bit. This is accomplished by coupling theanalog input signal to the input of a single comparator and usingcontrol logic to couple different reference values to another input ofthe comparator for the conversion of each successive bit of the outputrepresentation. Thus, serial A/D converters are characterized by theadvantages of relatively simple circuit configurations and low powerconsumption and by the disadvantage of slow conversion speeds. ParallelA/D converters, on the other hand, use a plurality of comparators eachwith its own distinct input reference value to allow all bits of thedigital output representation to be produced simultaneously. Althoughthe approach offers the advantage of minimum conversion time, this highspeed operation is obtained only with greatly increased circuitcomplexity and component count. Thus, for binary output representations,N output bits would require 2^(N) separate comparators. Many potentialapplications for high speed A/D converters require digital outputrepresentation of 8-bits. In applications where an A/D converterinterfaces with a bus-oriented microprocessor system, an 8-bit output(which can then correspond to a computer data byte) is particularlyuseful. Similarly, the encoding of samples representative of a 6megahertz color video signal requires a converter of at least 8-bits andconversion times of 80 nanoseconds or less. The implementation of an A/Dconverter having an 8-bit output using the parallel approach wouldrequire 256 separate comparator circuits. An A/D converter of thiscomplexity and high component count is impractical from a cost point ofview and, in particular, precludes implementation in monolithicintegrated circuit form.

Another approach to A/D conversion is the serial-parallel A/D converterwhich is a merger of the two conversion techniques previously discussed.In converters of this type, the digital output representation ispartitioned into a most significant bit (MSB) group and a leastsignificant bit (LSB) group. Parallel conversion is used to obtain thedigital output representations of both the MSB group and the LSB groupbut these conversions are performed in a serial fashion with the initialrepresentation of the MSB group determining the reference voltage valuerequired for the parallel conversion of the LSB group. Serial-parallelA/D converters yield faster conversion speeds than the all serial A/Dconverters with a circuit configuration which is simpler and has lowerpower consumption than the all parallel A/D converter.

Prior art serial-parallel A/D converters perform a conversion in thefollowing sequence:

1. Parallel A/D conversion is used to obtain a digital representation ofthe MSB group;

2. Digital-to-analog conversion is used to convert the MSBrepresentation back to a analog signal;

3. The analog representation of the MSB group is either subtracted fromthe input analog signal or is used to generate a change in voltagereference such that an analog difference signal is produced; and

4. Parallel A/D conversion is used to convert the analog differencesignal to obtain a digital representation of the LSB group.

Although the serial-parallel A/D converter represents a compromise whichprovides conversion speeds which are faster than exclusively serial A/Dconverters and which have reduced circuit complexity and component countfrom exclusively parallel A/D converters, the circuit delay timesrequired in past converters of this type have made them too slow forvery high speed applications such as color video encoding or interfacingwith a high-speed microprocessor. Thus, it has remained a problem toobtain an effective design for an A/D converter which is simple enoughin circuit configuration and low enough in component count to beimplemented as a monolithic integrated circuit but which is also capableof high-speed operation.

Assignee's co-pending patent application No. 1 listed above describesone approach by which improved conversion speeds are obtained byoverlapping the conversion operation required to produce a MSB and LSBgroup in a serial-parallel A/D converter. In this approach, the activecomparator output associated with MSB group conversion is used to switchthe appropriate voltage reference value to the input of a voltagesubtractor to obtain the analog difference signal required for LSB groupconversion. In addition to the speed advantages inherent in the systemconfiguration itself, the approach obtains improved speeds through theuse of differential current switching both to obtain a high-speedlatching comparator and to rapidly switch voltage reference values asrequired for the voltage subtraction. Although this approach works well,alternative system designs are possible which supply the referencevalues required for generating the analog difference signal in adifferent way thereby eliminating the requirement for precisiondifferential current switches in the voltage subtractor.

SUMMARY

Accordingly, it is an object of this invention to provide an improvedserial-parallel A/D converter which obtains improved conversion speedsthrough the use of overlapping operations and in which the MSB groupcomparison generates a reference current whose magnitude isrepresentative of the reference voltage to be subtracted from the analoginput signal to obtain the analog difference signal required for the LSBgroup comparison.

It is a further object of this invention to provide an improvedserial-parallel A/D converter which is amenable to implementation as amonolithic integrated circuit.

Briefly described, the present invention is a serial-parallel A/Dconverter in which a first parallel A/D converter means uses acomparator coupled to an analog input signal to produce a quantizedsignal which is then encoded to produce the MSB group of the digitaloutput representation of the applied analog input. Precision currentmatching circuits responsive to the individual stages of the MSBcomparator couple to a current summing node and thereby establish areference current whose magnitude corresponds to the reference voltagevalue identified by the MSB group comparator. The reference current andthe applied analog input couple as inputs to a voltage subtractor whoseoutput is an analog difference signal. The analog difference signalprovides the input for a second parallel A/D converter means whoseoutput is the LSB group digital representation of the applied analoginput. The analog difference signal appears at the output of the voltagesubtractor simultaneously with the encoding of the digitalrepresentation of the MSB group such that the operation of the firstparallel A/D converter means and the operation of the second parallelA/D converter means is overlapped in time, thereby obtaining increasedconversion speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an analog-to-digital converter according tothe invention.

FIG. 2 is a circuit schematic diagram of a comparator which can be usedto provide comparator 12 shown in the embodiment of FIG. 1.

FIG. 3 is a circuit schematic diagram of a voltage subtractor circuitwhich can be utilized to provide voltage subtractor 14 shown in FIG. 1.

FIG. 4 is a detailed circuit diagram of the precision current sourceshown in FIG. 2.

FIG. 5 is a representation circuit schematic diagram of the currentsplitting networks shown in FIG. 2.

FIG. 6 is a representation circuit schematic diagram of the voltagelevel shifting networks shown in FIG. 2.

FIG. 7 is a timing diagram which is useful in describing the operationof the invention.

FIG. 8 is a circuit schematic diagram of a comparator which can be usedto provide comparator 46 shown in the embodiment of FIG. 1.

FIG. 9 is a circuit schematic diagram of an encoder which can beutilized to provide both encoders shown in FIG. 1.

FIG. 10 is a logic diagram of output latches which can be utilized inthe embodiment of FIG. 1.

FIG. 11 is a circuit diagram of the compare-latch command generator ofFIG. 1.

FIG. 12 is a logic diagram of the read-latch command generator of FIG.1.

DETAILED DESCRIPTION

FIG. 1 shows a serial-parallel A/D converter 10 according to the presentinvention. FIG. 1 is a functional block diagram and, as such, a singleline interconnecting functional blocks may represent a single conductoror a multiplicity of conductors as will be made clear by the subsequentdetailed description. Power supply and reference voltage connections arealso omitted from FIG. 1 in the interest of simplicity.

In FIG. 1, an analog input couples to a first comparator 12 and avoltage subtractor 14 via conductor 16. The output of comparator 12couples to ROM encoder 18 via line 20, which represents a multiplicityof conductors. A reference current generated within comparator 12 iscoupled to voltage subtractor 14 via conductor 22. ROM encoder 18couples to output latch 24 via conductors 26, 28, 30 and 32. Outputlatch 24 produces a digital output representation on conductors 34, 36,38 and 40 which form the most significant bit (MSB) group output of thefirst parallel A/D converter means 42 formed by comparator 12, ROMencoder 18 and output latch 24. A second parallel analog-to-digitalconverter means 44 is formed by comparator 46, ROM encoder 48 and outputlatch 50. Conductor 52 couples the analog difference output signal ofvoltage subtractor 14 to an input of comparator 46. Line 54, whichrepresents a multiplicity of conductors, couples the output ofcomparator 46 to the input of ROM encoder 48. Conductors 56, 58, 60 and62 couple the outputs of ROM encoder 48 to the inputs of output latch50. The outputs of output latch 50 are coupled to conductors 64, 66, 68and 70 to form the least significant bit (LSB) group output ofanalog-to-digital converter circuit 10. Conductor 72 couples a clockinput to a compare-latch command generator 74, and line 76, whichrepresents a multiplicity of conductors, couples the output ofcompare-latch command generator 74 to the input of first comparator 12,the input of second comparator 46 and the input of read-latch commandgenerator 78. Line 80, which represents a multiplicity of conductors,couples the output of read-latch command generator 78 to the inputs ofoutput latch 24 and output latch 50.

In order to explain the operation of serial-parallel A/D converter 10and the particular advantages it offers for high-speed operation, ageneral overview of the functional operation of each of the elements ofFIG. 1 will be given. As described in the Background of the Inventionportion above, serial-parallel A/D converters make use of one parallelconverter to obtain the MSB group digital representation and a secondparallel converter to obtain the LSB group digital representation. Thus,FIG. 1 shows a first converter 42 producing the MSB group representationon the conductors 34, 36, 38 and 40, and a second converter 44 producingthe LSB group representation on conductors 64, 66, 68 and 70. Each ofthese converters contains similar functional elements. In converter 42 acomparator 12 compares the analog input on conductor 16 with a series ofinternal reference voltage values to produce a comparator output on oneof the conductors forming line 20 which, in this case, representssixteen separate comparator output conductors. These comparator outputconductors provide the input to ROM encoder 18. ROM encoder 18 respondsto the particular active comparator output to produce a digitalrepresentation; for example, a 4-bit binary representation, onconductors 26, 28, 30 and 32 which are then buffered by output latch 24whose outputs are the MSB group digital representation. Parallelconverter 44 contains similar functional elements in comparator 46, ROMencoder 48 and output latch 50, except that comparator 46 responds tothe analog difference signal produced by voltage subtractor 14 ratherthan the analog input.

The high-speed analog-to-digital conversion obtainable with theconfiguration of FIG. 1 derives from the individual functional elementswhich are individually adapted for high-speed operation and, moreparticularly, from the particular system arrangement which provides fora time overlap in the operation of the second parallel converter means44 with respect to the first parallel converter means 42. Thus, as isdescribed in greater detail below, the reference current produced bycomparator 12 allows the analog difference signal produced by voltagesubtractor 14 on conductor 52 to initiate the action of the secondparallel converter means 44 concurrently with the action of ROM encoder18 and output latch 24. The basic timing for these concurrent operationsis provided by compare-latch command generator 74 which provides"compare" and "latch" timing signals which couple to comparator 12 andcomparator 46. These timing signals also couple to read-latch commandgenerator circuit 78 which produces an output clock or strobe signal foroutput latch 24 and output latch 50, thereby synchronizing the operationof A/D converter circuit 10 such that the digital representation of theMSB group and the LSB group are both available at the same time. Giventhis framework, the particular advantages of an A/D converter systemwhich provides high-speed operation through time overlapping parallelconversions using a reference current to provide a simple voltagesubtraction configuration will now be explained in terms of the detailedstructure and operation of the comparator circuit of FIG. 2 and thevoltage subtractor circuit of FIG. 3.

FIG. 2 shows one particular embodiment of a highspeed comparator circuitwhich can be used as comparator 12 in the A/D converter circuit ofFIG. 1. The basic function of the comparator circuit of FIG. 2 is tocompare the applied analog input voltage to a series of voltagereference values to produce a MSB group digital output. If the number ofbits in the MSB group digital output is designated M, the applied inputvoltage will be compared to 2^(M) -1 voltage reference values using2^(M) -1 comparator stages. Thus, for the 4 bit MSB group embodiment ofFIG. 2, the function is accomplished using fifteen comparator stages.Each of the comparator stages contains identical circuitry and, forsimplicity in the drawing, FIG. 2 shows the detailed circuit structureand interconnections of only three of these comparator stages which are,accordingly, representative of the others. Thus, as shown, the appliedanalog input signal couples via conductor 102 to a first comparatorstage 104, a second comparator stage 106 and additional 12 comparatorstages which are part of block 108 not shown specifically, and afifteenth comparator stage 110. The reference voltage values for eachcomparator stage are generated by a series connected resistive ladder.This ladder is formed by a first terminal 114 which couples to resistor115 which couples to terminal 116 which, in turn, couples to resistor117 which couples to terminal 118, etc. This series interconnectioncontinues for twelve additional resistors which are included in block108, but not shown, and then continues with a connection to resistor 121which couples to node 122 which couples to resistor 123 which couples toterminal 124. Thus, the connection of a minimum reference voltage valueto terminal 114 and a maximum voltage reference value to terminal 124produces a series of reference voltage values between the minimum andthe maximum at each of the recited nodes.

The structure and operation of comparator circuit 100 will now beexplained in terms of the structure and operation of individualcomparator stage 106. Comparator stage 106 comprises a constant currentsource transistor 125, a pair of common emitter input transistors 126and 127, a pair of regenerative latching transistors 128 and 129 and apair of compare-latch mode selection transistors 130 and 131. Theemitter of first current source transistor 125 couples to resistor 132which, in turn, couples to a power supply conductor 133 which couples tothe V- power supply. The base of constant current source transistor 125connects to conductor 134 which couples in common to the bases of eachof the first constant current source transistors in each of the othercomparator stages. Conductor 134 also couples to the output of precisionconstant current generator 135, which establishes the required operatingreference current level for each of the first constant current sourcetransistors connected to conductor 134.

FIG. 4 shows the detailed structure of precision constant currentgenerator circuit 135. A first operational amplifier 135A has a firstdifferential input which couples to reference voltage V_(R1) viaconductor 135B. A second differential input of operational amplifier135A couples to the amplifier output via conductor 135C which alsocouples to a first terminal of resistor 135D. A second terminal ofresistor 135D couples to conductor 135E which couples to a firstdifferential input of a second operational amplifier 135F. A seconddifferential input of operational amplifier 135F couples to referencevoltage V_(R2) via conductor 135G. The output of operational amplifier135F couples to conductor 134 which couples to the base of transistor135H and the base of transistor 135I and which forms the output ofprecision constant current generator circuit 135 as shown in FIG. 2. Theemitters of transistors 135H and 135I couple to V- power supplyconductor 133 via resistors 135J and 135K, respectively. The collectorof transistor 135H couples to the emitter of transistor 135L whose basecouples to the collector of transistor 135I and whose collector couplesto the emitter of transistor 135M. The collector of transistor 135Mcouples to conductor 135E. The base of transistor 135L couples to thecathode of diode 135N whose anode couples to the base of transistor 135Mand to reference voltage value V_(R2).

In general, the voltage difference between the voltages applied to theinputs of operational amplifiers 135A and 135F is 2 V_(rd) where V_(rd)is the voltage difference between adjacent reference voltages. For acomparator having M output bits,

    V.sub.rd =(1/2.sup.M)[(V.sub.REF).sub.MAX -(V.sub.REF).sub.MIN ]

As a result, the circuit of FIG. 4 generates a constant current:

    I.sub.c =2V.sub.rd /R1

where R₁ =the resistance value of resistor 135D. The current sources ineach individual comparator stage such as transistor 125 and precisionresistor 132 (having value R1) in comparator stage 106 each generate anidentical current I_(c). The collector of constant current sourcetransistor 125 couples to conductor 138 which in turn couples to theemitters of compare-latch mode selection transistors 130 and 131. Thebase of transistor 130 couples to conductor 139 which is the comparemode clock conductor which couples in the same manner to each of theindividual comparator stages. The base of transistor 131 couples toconductor 140 which is the latch mode clock conductor which couples toeach of the individual comparator stages. The collector of transistor130 couples to conductor 141 which in turn couples to the emitters ofinput transistors 126 and 127. The collector of transistor 131 couplesto conductor 142 which, in turn, couples to the emitters of regenerativelatching transistors 128 and 129. The collector of input transistor 126couples to conductor 143 which couples to the collector of regenerativelatching transistor 129, a first terminal of level shifting circuit 128Aand a first terminal of current splitting circuit 144. The collector ofinput transistor 127 couples to conductor 145 which couples to thecollector of regenerative latching transistor 128, a first terminal oflevel shifting circuit 129A and to a first terminal of current splittingcircuit 146. A second terminal of current splitting circuit 146 couplesto a first emitter of output transistor 147. A second emitter of outputtransistor 147 couples to current splitting circuit 148 which is part ofcomparator stage 104. A second terminal of current splitting circuit 144couples to a first emitter of output transistor 149. A second emitter ofoutput transistor 149 couples to the current splitting circuit of asimilar comparator stage (not shown) within block 108.

A third terminal of current splitting network 144 couples to conductor144A which is a first reference current or first D/A current summingoutput of comparator circuit 100. Conductor 144A couples to a thirdterminal of current splitting network 148 in comparator stage 104, to athird terminal of current splitting network 144B in comparator stage 110and to similar current splitting network third terminals (not shown) ineach of the comparator stages represented by block 108. Similarly, athird terminal of current splitting network 146 couples to conductor146A which is a second reference current or second D/A current summingoutput of comparator circuit 100. Conductor 146A couples to a thirdterminal of current splitting network 146B in comparator stage 104, to athird terminal of current splitting network 146C in comparator stage 110and to similar current splitting network third terminals (not shown) ineach of the comparators stages represented by block 108.

FIG. 5 shows the detailed circuit structure of a particular currentsplitting network 148. This structure and its corresponding operation isidentical to the other current splitting networks of comparator circuit100 such as current splitting network 144, 146, etc. In currentsplitting network 148, the emitter of transistor 148A couples to thefirst terminal of a resistor 148B and the emitter of transistor 148Ccouples to the first terminal of resistor 148D. The second terminal ofresistor 148B and the second terminal of resistor 148D couple togetherto form a first terminal of current splitting network 148. The collectorof transistor 148A couples to conductor 148E which couples to the baseof transistor 148A, the base of transistor 148C and the emitter oftransistor 148F. The base of transistor 148F couples to the collector oftransistor 148C and to a first terminal of resistor 148J whose secondterminal couples to conductor 148H which forms a second terminal ofcurrent splitting network 148. The collector of transistor 148F couplesto conductor 148G which forms a third terminal of current splittingnetwork 148. The function of the current splitting network is toproperly distribute precision matched currents to the D/A currentsumming outputs and at the same time allow the proper single high levelcomparator output (described below) to be generated. Thus, currentsplitting networks are inserted in both the left hand and right handsides of each comparator stage as shown in FIG. 2. In the currentsplitting network as represented by FIG. 5, the transistors 148A, 148Cand 148F are all identical and resistor 148B is equal to resistor 148D.Denoting the split current in the third terminal formed by conductor148G as I₁ and the split current in the second terminal formed byconductor 148H as I₂, and denoting the current gain of transistors 148A,148C and 148F by β, the resulting relation between the split currentsis: ##EQU1## or, for β>100:

    I.sub.2 /I.sub.1 ≃1

In the network, resistors 148B and 148D serve to linearize the devicecharacteristics by minimizing any base emitter voltage mismatchesbetween transistors and by increasing the differential gain duringcomparison.

Turning again to FIG. 2, the base of output transistor 147 and the baseof output transistor 149 are each coupled to an output bias voltageconductor 150 which also couples to the bases of similar outputtransistors for each of the individual comparator stages. This outputbias voltage is generated by circuit 151 in which the emitter oftransistor 152 is coupled via resistor 153 to ground and the base oftransistor 152 couples to the collector of transistor 152 and to V+power supply conductor 154 via resistor 155. The collector of transistor152 couples to the base of transistor 156 whose collector couples to theV+ power supply via conductor 154 and whose emitter couples to conductor150 and to resistor 157 which in turn couples to ground.

Referring again to representative comparator stage 106 in FIG. 2, thecollector of output transistor 147 couples to conductor 158 which is oneof the multiplicity of conductors forming comparator output 20previously mentioned in the discussion for FIG. 1. Conductor 158 alsocouples to a first terminal of an output resistor 159 whose secondterminal couples to V+ power supply conductor 154. In a similar manner,the collector of output transistor 149 couples to conductor 160 which isanother of the multiplicity of conductors forming the output of thecomparator circuit. Conductor 160 also couples to a first terminal ofresistor 161 whose second terminal couples to V+ power supply conductor154.

Turning now to the operation of the comparator circuit of FIG. 2, astypified by the operation of representative comparator stage 106, itshould first be noted that the comparator stage has two fundamentalmodes of operation, the compare mode and the latch mode. The comparemode is defined by a high level on the compare clock conductor 139 and alow level on the latch clock conductor 140. Conversely, the latch modeis defined by a low level on conductor 139 and a high level on conductor140. The clock timing waveforms of FIG. 7 illustrate the relationshipbetween these two clock signals. The compare-latch mode selectingtransistor pair 130 and 131 determines whether comparator stage 106 isin the compare mode or the latch mode by supplying the current valuedefined by constant current transistor 125 to the emitters of either the"compare" emitter-coupled transistors 126 and 127 or to the "latch"emitter-coupled transistor pair 128 and 129. Since the clock signals onconductors 139 and 140 are complements (see FIG. 7) only the comparetransistors or the latch transistors will be active at any one time. Inthe compare mode, the analog input voltage present on conductor 102,which couples to the base of transistor 127, is compared to thereference voltage value present on conductor 118 which couples to thebase of transistor 126. The coupling between the emitters of transistors126 and 127 form a differential amplifier. When the applied analog inputvoltage on conductor 102 is more than the reference voltage value onconductor 118, the collector current of transistor 127 will be greaterthan the collector current of transistor 126 and, as a result, a plus tominus differential voltage will be produced between conductors 143 and145. Conversely, if the applied analog input voltage coupled to the baseof transistor 127 is less than the reference voltage applied to the baseof transistor 126, a minus to plus differential voltage will be producedbetween conductors 143 and 145. Thus, in the compare mode the polarityof the differential voltage developed between the collector oftransistor 126 and the collector of 127 indicates whether the appliedanalog input signal applied to comparator stage 106 is greater than, orless than, the reference voltage value applied to that same stage.

The second terminal of level shifting network 128A couples to the baseof regenerative latching transistor 128 and also couples to thecollector of current source transistor 128B. In a similar fashion, asecond terminal of level shifting network 129A couples to the base ofregenerative latching transistor 129 and also couples to the collectorof current source transistor 129B. The bases of constant current sourcetransistors 128B and 129B couple to conductor 134A which is the outputof constant current generator circuit 136A. In constant currentgenerator 136A, conductor 134A couples to the base of transistor 136Bwhich in turn couples to the collector of transistor 136B and to a firstterminal of resistor 136C. A second terminal of resistor 136C couples toground. The emitter of transistor 136B couples to a first terminal ofresistor 136D whose second terminal couples to the V- power supply. Theemitters of constant current source transistors 128B and 129B couple tothe V- power supply via resistors 128C and 129C respectively. A thirdterminal of level shifting network 128A and a third terminal of levelshifting network 129A couple in common to V+ power supply conductor 154.

FIG. 6 shows the detailed circuit structure of a level shifting circuitsuitable for use as circuit 128A or circuit 129A in the representativecomparator stage 106. The circuit uses three identical transistors. Thefirst terminal of the level shifting network couples to the base oftransistor 128D whose emitter couples to the base of transistor 128E andto a first terminal of resistor 128F. The emitter of transistor 128Ecouples to the base and collector of transistor 128G whose emitter formsthe second terminal of the level shifting circuit. The collector oftransistor 128D couples to the collector of transistor 128E to form thethird terminal of the level shifting network. When the level shiftingnetwork is arranged such that a constant current source is connected tothe second terminal and the third terminal is connected to a V+ powersupply, the circuit functions to provide a path for the constant currentto flow from the first terminal to the second terminal with a specifiedvoltage drop or level shift between the first terminal and the secondterminal. By this operation, saturation is prevented when the comparatorstages are in the latch mode, thereby increasing switching speed andalso preserving the summing current accuracy needed for correctfunctioning of the D/A summing current outputs.

A key feature in the speed of operation of each comparator stage and,thus, of the overall comparator circuit is the use of latchingtransistors 128 and 129. when the compare-latch clock signals switchsuch that the level of the compare clock conductor 139 goes from high tolow, the level of the latch clock conductor 140 goes from low to high(see FIG. 7). When the clock switches, transistor 130 switches to thenonconducting state and, simultaneously, transistor 131 switches to theconducting state such that latch transistors 128 and 129 are enabled.The regenerative cross-coupling formed by conductors 143 and 145 acts toreenforce the polarity of any differential voltage established betweenthe collectors of transistors 126 and 127 in the compare mode and, thus,store or "latch" the results of the comparison. Assume, for example,that the input voltage applied via conductor 102 is only slightlygreater than the reference voltage value on conductor 118 so that at theend of the compare interval the collector of transistor 126 is only afew millivolts more positive than the collector of transistor 127. Asthe circuit enters the latch mode interval and transistors 128 and 129begin to conduct, the slightly more positive voltage on conductor 143will cause greater conduction in transistor 128 and less conduction intransistor 128 with the result that conductor 143 will become even morepositive with respect to conductor 145. This regenerative effectproceeds rapidly until transistor 129 is nonconducting and transistor128 is conducting. The speed inherent in a differential amplifierconfiguration which makes use of current switching together with the"snap" action of a latch circuit, which uses regeneration to capture theresults of a fast comparison, are combined to provide a particularlyuseful high-speed comparator configuration.

High speed and a simple circuit configuration are further provided bythe comparator circuit of FIG. 2 through the use of multiple emitteroutput transistors such as transistor 147 and transistor 149. Each ofthe individual comparator stages functions in the manner previouslydescribed for representative stage 106 and in the latch interval, eachstage will assume one of two possible states defined by the presence orabsence of current in the current splitting load networks such asrepresentative networks 144 and 146. If, during the compare interval,the applied analog input voltage on conductor 102 was greater than thereference voltage value on conductor 118, during the latch mode network146 will conduct essentially all of the current flowing through constantcurrent source transistor 125 and network 144 will conduct essentiallyno current. Conversely, if the applied analog input voltage on conductor102 is less than the reference voltage value on conductor 118, network146 will conduct essentially no current.

Now consider the case where the applied analog input voltage has a valuewhich is greater than the reference voltage value on conductor 116 butless than the reference voltage value on conductor 118. During the latchinterval, this value of applied analog input voltage will result inessentially no current in network 146 and no current in network 148.This means that no current can flow in either emitter of transistor 147so that there is no current in the collector of transistor 147, andresistor 159 pulls output conductor 158 to a high level. Since allcomparator stages to the left of comparator stage 106 couple toreference voltage values which are greater than the applied analog inputvoltage on conductor 102, they will all be latched into a conductingstate which is the same as that described for stage 106 and, as aresult, the multiple emitter output transistors corresponding to thestages will each have at least one conducting emitter so that all of theoutput conductors of comparator output 20, except for conductor 158,will be held in the low state. Thus, the multiple emitter outputtransistor structure shown in FIG. 2 provides a simple and high-speedmethod for producing an output signal on a single one of a multiplicityof output conductors which indicates the value of an applied analoginput voltage with respect to a series of reference voltage values.

The overall effect of the current splitting networks such as currentsplitting network 144 and 146 of comparator stage 106 will now beconsidered. As noted above, each current splitting network generatesmatched currents and for each comparator stage the current splittingnetworks analogous to current splitting network 146 couples thesematched currents to conductor 146A which forms the D/A current summingoutput of comparator 100. For convenience in discussion, the current inconductor 146A is designated I_(A). In a similar fashion, the currentsplitting networks analogous to current splitting network 144 in eachcomparator stage couple in common to conductor 144A which forms thecomplementary D/A current summing output of comparator circuit 100. Forconvenience, the current in conductor 144A will be designated I_(B). Asnoted above, the current in each comparator stage is determined by thecollector current of a constant current source transistor analogous totransistor 125 in comparator stage 106. This constant current isdesignated I_(C). For each individual comparator stage, if the inputvoltage is larger than the reference voltage then the constant currentI_(C) conducts through the transistor analogous to transistor 127 incomparator stage 106 and, because of the current splitting, one-half ofthis current goes in the third terminal of current splitting networkanalogous to current splitting network 146 in comparator stage 106 tocontribute to the total D/A summing current in conductor 146A. Insimilar fashion, if the input voltage is smaller than the referencevoltage then one-half of the constant current I_(C) is contributed tothe I_(B) current flowing in conductor 144A.

The comparator circuit of FIG. 2 also includes an additional referencecurrent generating circuit useful for some inventive embodiments. Thiscircuit comprises constant current source transistor 125A having a basecoupled to current reference conductor 134, having an emitter coupled toa first terminal of precision resistor 132A and having a collectorcoupled to a first terminal of current splitting network 125B. Detailedcircuit structure of the current splitting network is shown in FIG. 5. Asecond terminal of precision resistor 132A couples to the V- powersupply conductor. A second terminal of current splitting network 125Bcouples to V+ power supply conductor 154 and a third terminal of currentsplitting network 125B couples to conductor 125C to form a referencecurrent output for the circuit. Because of the precision matching oftransistor 125A and resistor 132A with the similar constant currentcircuits in each of the comparator stages, the current in the collectorof transistor 125A has the value I_(c) and, because of the currentsplitting network 125B, the resultant current in output conductor 125Chas the value Ic/2. This additional reference current is useful forcertain system configurations employing the comparator of FIG. 2.

Thus, I_(A) is the sum of the comparator currents where the inputvoltage is larger than the reference voltage, and I_(A) thereforerepresents a count of all the digital levels less than the input level.Similarly, I_(B) represents a count of all digital levels above theinput voltage. These currents thus constitute the D/A outputs needed fora subsequent serial stage of A/D conversion which can be subtraction orreference voltage level shifts. The use of comparator stages whichinclude current splitting networks analogous to network 144 combinedwith level shifting networks analogous to 128A are of particularadvantage in obtaining high-speed conversion in a parallel-serial A/Dconverter. The advantage is derived from the fact that the D/A currentsumming outputs described above are generated as part of the currentswitching inherent in the operation of the latching comparator. The highspeed inherent in current switching of this type is further enhancedthrough the use of level shifting circuits analogous to level shiftingcircuit 128A to prevent saturation and to eliminate distortion in theconstant current value being switched. The availability of precision D/Acurrent sum outputs make it possible to implement parallel-serial A/Dconverters which operate at high speeds by using relatively simplecircuit structures to accomplish the voltage subtraction or referencevoltage level shifting operations required for serial operations.

As previously discussed, the key to the high A/D conversion speedsobtained by the present invention is the time overlap or coincidence ofthe parallel conversion of the MSB group digital output and the LSBgroup digital output. This time overlap is obtained by usingintermediate outputs obtained during the process of the MSB groupconversion to determine the particular reference voltage value whichmust be subtracted from the applied analog input to obtain the analogdifference signal required for the parallel conversion of the LSB groupoutput. Assignee's co-pending patent application No. 1 listed abovedescribes one approach by which improved conversion speeds are obtainedby overlapping. The approach of the present invention differs in thatthe comparator circuit used for the parallel conversion of the MSB groupdigital output includes a D/A current summing output I_(A) whosemagnitude represents a count of all the reference voltage values whichare less than the applied input value.

Thus, in summary, the comparator of FIG. 2 simultaneously performs theflash encoding of the first most significant bits (i.e., a digitaloutput) and also generates an analog representation of this digitalsignal which is suitable for subtraction or level shifting (an analogoutput). The comparator may be described as follows:

1. Number of bits=M

2. Assume input voltage is between the two reference voltages associatedwith the n^(th) comparator stage:

    V.sub.r(n) <V.sub.in <V.sub.r(n+1).

3. Number of voltage comparators=2^(M) -1.

4. The voltage difference between two adjacent references:

    V.sub.rd =1/2.sup.M [(V.sub.REF).sub.MAX -(V.sub.REF).sub.MIN ].

5. The constant current in each comparator:

    I.sub.c =2V.sub.rd /R1.

6. The input voltage V_(in) is:

    (n+1)V.sub.rd +(V.sub.REF).sub.MIN >V.sub.in >nV.sub.rd +(V.sub.REF).sub.MIN

7. The summing current I_(A) represents the D/A current output:

    I.sub.A =n(I.sub.C /2)=n(V.sub.rd /R1).

8. The summing current I_(B) represents the complement of the D/Aoutput: ##EQU2##

9. The sum of the summing currents forms a constant current:

    I.sub.A +I.sub.B =constant=(2.sup.M -1)(I.sub.C /2)

The availability of the summing current output I_(A) offers asignificant advantage in that high-speed operation can be obtained witha very much simpler voltage subtractor configuration. FIG. 3 shows sucha voltage subtractor circuit 300 suitable for use in the presentinvention. As shown, the voltage subtractor consists of a current mirrorcircuit 301, an operational amplifier 302 and a feedback resistor 303.For the particular embodiment of FIG. 3, current mirror circuit 301 isimplemented using PNP transistors. Conductor 304 couples the appliedanalog input voltage V_(in) to one input of operational amplifier 302.Conductor 304 is equivalent to conductor 16 shown in FIG. 1 and toconductor 102 shown in FIG. 2. Conductor 305 couples the summing currentoutput of the comparator to a first terminal of current mirror circuit301. Conductor 305 is equivalent to conductor 22 shown in FIG. 1 or toconductor 146A shown in FIG. 2. In current mirror circuit 301, conductor305 couples to the base of transistor 306 and to the cathode of diode307. The anode of diode 307 couples to the collector of transistor 308whose emitter couples to V+ power supply conductor 309 which alsocouples to the emitter of transistor 310. Conductor 309 is equivalent toV+ power supply conductor 154 shown in FIG. 2. The collector oftransistor 310 couples to its own base, the base of transistor 308, andto emitter of transistor 306. The collector of transistor 306 couples toconductor 311 which forms a second terminal of current mirror circuit301. Conductor 311 couples to a second differential input of operationalamplifier 302 and to a first terminal of feedback resistor 303. A secondterminal of feedback resistor 303 couples to conductor 312 which couplesto the output of operational amplifier 302 and forms the V_(O) output ofvoltage subtractor circuit 300. Conductor 312 corresponds to conductor52 shown in FIG. 1.

Because of the action of current mirror circuit 301, the current inconductor 311 is identical to the current in conductor 305. Since thedifferential input of operational amplifier 302 coupled to conductor 311conducts essentially no current, the I_(A) summing current in conductor305 results in a equal current in resistor 303. When the value ofresistor 303 is chosen to have the value R1 defined above, theinput-output relationship for voltage subtractor circuit 300 is given bythe equation:

    V.sub.O =V.sub.in -I.sub.A R1=V.sub.in -nV.sub.rd

Thus, it is seen that the simple and direct subtractor circuit of FIG. 3produces a analog difference voltage V₀ as required for the LSBconversion in the serial-parallel conversion thereby providingoverlapping operations for increased conversion speeds with a minimum ofadditional circuit complexity.

FIG. 8 shows one particular embodiment of a high-speed comparatorcircuit which can be used as comparator 46 in the A/D converter circuitof FIG. 1. The basic function of the comparator circuit of FIG. 8 is tocompare the applied analog input voltage to a series of voltagereference values to produce a digital output. For the particularembodiment of FIG. 8, this is accomplished using fifteen comparatorcircuits. Each of the comparator circuits contains identical circuitryand, for simplicity in the drawing, FIG. 8 shows the detailed circuitstructure and interconnections of only four of these comparator circuitswhich are, accordingly, representative of the others. Thus, as shown,the applied analog input signal couples via conductor 102' to a firstcomparator state 104', a second comparator stage 106', an additional 11comparator stages which are part of block 108' but not shownspecifically, a fourteenth comparator stage 110', and a fifteenthcomparator stage 112'. The reference voltage values for each comparatorstage are generated by a series connected resistive ladder. Theconnection of a minimum reference voltage value to terminal 114' and amaximum voltage reference value to terminal 124' produces a series ofreference voltage values between the minimum and the maximum at each ofthe recited nodes.

Turning now to the operation of the comparator circuit of FIG. 8, astypified by the operation of representative comparator stage 106, itshould first be noted that the comparator stage has two fundamentalmodes of operation, the compare mode and the latch mode. The comparemode is defined by a high level on the compare clock conductor 139' anda low level on the latch clock conductor 140'. Conversely, the latchmode is defined by a low level on conductor 139' and a high level onconductor 140'. The clock timing waveforms of FIG. 7 illustrate therelationship between these two clock signals. The compare-latch modeselecting transistor pair 130' and 131' determines whether comparatorstage 106' is in the compare mode or the latch mode by supplying thecurrent value defined by constant current transistor 125' to theemitters of either the "compare" emitter coupled transistors 126' and127' or to the "latch" emitter-coupled transistor pair 128' and 129'.Since the clock signals on conductors 139' and 140' are complements (seeFIG. 7) only the compare transistors or the latch transistors will beactive at any one time. In the compare mode, the analog input voltagepresent on conductor 102' which couples to the base of transistor 127'is compared to the reference voltage value present on conductor 118'which couples to the base of transistor 126'. The coupling between theemitters of transistors 126' and 127' form a differential amplifier.When the applied analog input voltage on conductor 102' in less than thereference voltage value on conductor 118', the collector current oftransistor 127' will be greater than the collector current of transistor126' and as a result the voltage developed across resistor 146' will belarger than the voltage developed across resistor 144'. Conversely, ifthe applied analog input voltage coupled to the base of transistor 127'is less than the reference voltage applied in the base of transistor126', the voltage developed across resistor 144' will be greater thanthe voltage developed across resistor 146'. Thus, in the compare modethe polarity of the differential voltage developed between the collectorof transistor 126' ands the collector of 127' indicates whether theapplied analog input signal applied to comparator stage 106' is greaterthan or less than the reference voltage value applied to that samestage. The regenerative cross-coupling formed by conductors 143' and145' acts to re-enforce the polarity of any differential voltageestablished between the collectors of transistors 126' and 127' in thecompare mode and thus store or "latch" the results of the comparison.The speed inherent in a differential amplifier configuration which makesuse of current switching together with the "snap" action of a latchcircuit which uses regeneration to capture the results of a fastcomparison are combined to provide a particularly useful high-speedcomparator configuration.

High speed and a simple circuit configuration are further provided bythe comparator circuit through the use of multiple emitter outputtransistors such as transistor 147' and transistor 149' shown in FIG. 8.Each of the individual comparator stages function in the mannerpreviously described for representative stage 106, and in the latchinterval each stage will assume one of two possible states defined bythe presence or absence of current in the load resistors such asrepresentative resistors 144' and 146'. If, during the compare interval,the applied analog input voltage on conductor 102' was greater than thereference voltage value on conductor 118' during the latch mode,resistor 146' will conduct essentially all of the current flowingthrough constant current source transistor 125' and resistor 144' willconduct essentially no current. Conversely, if the applied analog inputvoltage on conductor 102' is less than the reference voltage value onconductor 118', resistor 146' will conduct essentially no current. Nowconsider the case where the applied analog input voltage has a valuewhich is greater than the reference voltage value on conductor 116' butless than the reference voltage value on conductor 118'. During thelatch interval this value of applied analog input voltage will result inessentially no current in resistor 146' and no current in resistor 148'.This means that no current can flow in either emitter of transistor 147'so that there is no current in the collector of transistor 147' andresistor 159' pulls output conductor 158' to a high level. Since allcomparator stages to the left of comparator stage 106' see referencevoltage values which are greater than the applied analog input voltageon conductor 102' they will all be latched into a conducting state whichis the same as that described for stage 106' and as a result themultiple emitter output transistors corresponding to these stages willeach have at least one conducting emitter so that all of the outputconductors of comparator output 20' except for conductor 158' will beheld in the low state. Thus, the multiple emitter output transistorstructure shown in FIG. 8 provides a simple and high-speed method forproducing an output signal on a single one of a multiplicity of outputconductors which indicates the value of an applied analog input voltagewith respect to a series of reference voltage values.

The comparator circuit shown in FIG. 8 also includes a bias generatingcircuit 165' which is used for generating reference bias voltages usedby the read-only memory encoder circuit (FIG. 9).

FIG. 9 shows one particular embodiment of a read-only memory encodercircuit which can be used to implement the A/D converter of the presentinvention. Read-only memory encoder circuit 200 comprises a read-onlymemory input section 202, an output section 104 and a bias voltagegenerator 206. Read-only memory input section 202 consists of aplurality of multiple emitter transistors. The number of multipleemitter input transistors corresponds to the number of digital outputsproduced by the comparator circuit of the A/D converter. Thus, for theparticular embodiment of FIG. 9, read-only memory input section 202comprises sixteen multiple emitter input transistors. Transistors 208,209, 210 and 211 are representative of these transistors as shown inFIG. 9, the collectors of all of the multiple emitter input transistorsare coupled to a common V+ power supply conductor 212. Each of theconductors forming the plurality of digital outputs from the comparatorcircuit of FIG. 8 couples to the base of one multiple emitter inputtransistor. As previously discussed, the comparator circuit (FIG. 8)produces a plurality of digital output signals each of which representsa particular level of applied analog input signal. The particulararrangement of connections to the multiple emitters of the inputtransistors provide an encoding function by which a unique digital codeis obtained for each comparator output signal and, thus, for eachparticular level of applied input signal. This arrangement of emitterconnections is accomplished by providing a plurality of commonconductors such as conductors 217, 218, 219 and 220. For the particularembodiment of read-only memory encoder 200, a binary code is used. Thus,the comparator output signal on conductor 213, which indicates that theapplied analog input signal is greater than a first reference voltagevalue, couples to the base of multiple emitter transistor 208, which hasnone of its emitters coupled to the common conductors 217, 218, 219 and220 and thus defines a binary code 0000. A comparator output signal onconductor 214, which indicates that the level of applied analog inputvoltage is greater than a second reference voltage value, coupled to thebase of multiple emitter transistor 209 which has a first emitter 221coupled to common conductor 220 thereby defining the binary code 0001.Similar emitter connections are used for the other multiple emittertransistors shown in read-only memory input circuit 202. A multipleemitter encoding arrangement such as shown in FIG. 9 is advantageousbecause coded representations of the applied analog input level, otherthan the binary code shown, can be readily obtained by simply changingthe emitter interconnection arrangement used. In a monolithic integratedcircuit, device structures incorporating several emitter device regionswithin a common base region or other similar techniques commonlyemployed in integrated circuit read-only memories provide a simple wayto obtain the multiple emitter structure shown and make it easy tochange the interconnection pattern as required by a new code.

The coded representation provided by the signals on each of theconductors 217, 218, 219 and 220 are coupled respectively to outputcircuits 226, 227, 228 and 229 which provide the amplification and levelconversion required by the output latch circuit (FIG. 10 describedbelow). Each of these output circuits operates using two constantcurrent source transistors. The reference current for these constantcurrent source transistors is provided by transistor 230 whose emittercouples to V- power supply conductor 231 via resistor 232 and whose baseand collector are coupled to conductor 233. Conductor 233 couples to thebases of the constant current source transistors of each output circutand to a first terminal of resistor 234 whose second terminal couples toa power supply terminal (GND) which is ground for the particularembodiment of FIG. 9. Conductor 233 also couples to the base of constantcurrent source transistor 235 which is part of reference voltagegenerating circuit 206.

As previously discussed, the reference bias voltage on conductor 172'defines a level midway between the low and high levels of the comparatoroutput, and reference voltage generating circuit 206 translates thisvoltage to the bias voltage present on conductor 237 thereby providing atrip point for high-speed current switching within each of the outputcircuits. The structure and operation of each of the output circuits isidentical and will be described in terms of the operation of outputcircuit 229.

The heart of output circuit 229 is the differential amplifier formed bytransistor 241A and 242. When the voltage at the base of transistor 241Ais less than the voltage at the base of transistor 242, the constantcurrent value established by constant current source transistor 244 andresistor 245 will be conducted by transistor 242, and the resultingvoltage drop across resistor 252 will establish a low level on outputconductor 251. As previously discussed, the operation of the comparatorcircuit results in a high level on one of the comparator outputconductors, for example, conductor 214. The encoding function of theemitter connections of each of the multiple emitter transistors steerssuch a high level to the inputs of the appropriate output drivercircuits. Thus, a high level on conductor 214 results in a high level atemitter 221 of multiple emitter transistor 209 which couples viaconductor 220 to the input of output circuit 229. This high level islevel shifted by transistor 250 and transistor 246 to produce theappropriate change in voltage at the base of transistor 241A. Transistor246 and transistor 250 operate in an emitter base reverse breakdownmode, and resistor 249 is chosen such that a small constant current isconducted by constant current source transistor 248. This small currentestablishes a stable value of reverse breakdown voltage for transistor246. As a result of the level shifted high level appearing at the baseof transistor 241A, transistor 241A conducts the constant current valueset by transistor 244, and transistor 242 becomes nonconducting so thatthe output level on conductor 251 changes from a low to a high level.

Circuit structures shown in FIG. 9 are particularly adapted to minimizethe effects of temperature and process variations in the monolithicintegrated circuit embodiment. The use of a current source, such ascurrent source transistor 248, to establish a small stable current valueto sustain the emitter base reverse breakdown voltage of the levelshifter transistors tends to eliminate the effects of processvariations. Also, the use of a common current source reference(conductor 233) for all output stages and in particular for thegeneration of the bias voltage on conductor 237 compensates for theeffects of temperature and process variation. It also should be notedthat transistor 250 can be eliminated in those applications where theadditional level shifting it provides is not required.

The operation of each of the output circuits of read-only memory encoder200 is the same as has been described for output circuit 229. Thusoutput circuit 228 responds to a high level on conductor 219 to producea level shift at high level on output conductor 252, output circuit 227responds to a high level on conductor 218 to produce a level shift athigh level on output conductor 253 and output circuit 226 responds to ahigh level on conductor 217 to produce a level shifted high level onoutput conductor 254.

FIG. 11 shows a compare-latch command generator circuit 500 which can beused to implement the present invention. This circuit uses high-speedcurrent switching to provide the high frequency compare and latch modeclock signals required by the comparator to accomplish the high-speedA/D conversion objectives of the present invention. As shown in FIG. 11,a clock input is level shifted by a zener diode 501 and coupled to afirst differential amplifier formed by transistors 502 and 503. Thecollector of transistor 503 couples to Q1 output conductor 504 which isthe compare mode clock for the most significant bit (MSB) comparator.Similarly, the collector of transistor 502 couples to the Q1 outputconductor 505 which is the latch mode clock for the most significant bit(MSB) comparator. The level shifted clock input also couples viaconductor 506 to resistor 507 and capacitor 508. Resistor 507 andcapacitor 508 form a delay line which delay the applied clock inputprior to its being coupled to the differential amplifier formed bytransistors 509 and 510. The collector of transistor 510 couples to Q2clock conductor 511 which is the compare mode clock for the leastsignificant bit comparator. Similarly, the collector of transistor 509couples to the Q2 clock conductor 512 which is the latch mode clock forthe least significant bit comparator. It should also be noted in FIG. 11that the collector load resistors of transistors 502 and 503 couple toground power supply conductor 513 via transistor 514 while the collectorload resistors of transistor 509 and 510 couple to V+ power supplyconductor 515 via transistor 516. These differences in collector supplyvoltage provide compare mode and latch mode clock signals appropriatefor each comparator. The time relationship of the compare mode and latchmode outputs of compare-latch command generator circuit 500 are shown inthe timing diagram of FIG. 7.

In order that all bits of the digital representation appearsimultaneously, the A/D converter circuit of the present inventionincludes an output latch circuit 600 and an output read/latch commandgenerator circuit 700.

Referring to FIG. 10, output latch circuit 600 comprises four identicallatch circuits 601, 602, 603 and 604. The structure and operation ofeach latch circuit will be explained in terms of representative circuit601 which comprises NAND gate 605, 606 and 607. NAND gates 606 and 607are cross-coupled to form a latch. Conductor 608 couples the φR clocksignal generated by output read-latch command generator circuit 700 toan input of NAND gate 607. This signal forces the latch formed by NANDgate 606 and 607 to the "reset" state during the time interval T₄ shownin FIG. 7. Conductor 609 couples the φR clock signal generated by outputread-latch command generator circuit 700 to an input of NAND gate 605.Conductor 609 is high during the time interval t₄ shown in FIG. 7 andthus enables NAND gate 605 during that time such that the digital datacoupled to a second input of NAND gate 605 via conductor 610 is invertedand applied to the "set" input of the latch formed by NAND gates 606 and607. At the end of time interval t₄, the φR clock signal on conductor608 returns to a high level such that the latch circuit formed by NANDgates 606 and 607 is no longer held in the reset state and will,accordingly, latch into the state defined by the logic level at theoutput of NAND gate 605 thereby producing the required digital output onconductor 611 which couples to the output of NAND gate 606. In a similarfashion the ROM encoder output applied to latch circuit 602 viaconductor 612 is latched to produce a digital output on conductor 613;the ROM encoder output applied to latch circuit 603 via conductor 614 islatched to produce a digital output on conductor 615; and the ROMencoder output applied to latch circuit 604 via conductor 616 is latchedto produce a digital output on conductor 617.

FIG. 12 shows output read-latch command generator circuit 700 whichcomprises cross-coupled NOR gates 701 and 702, NAND gate 703 andinverter 704. The inputs to circuit 700 are clock signals from thecompare-latch command generator circuit 500 shown in FIG. 11. Comparemode clock φ1 couples via conductor 504 to an input of NOR gate 701, andcompare mode clock φ2 couples to an input of NOR gate 702 via conductor511. These signals alternately set and reset the latch formed by NORgates 701 and 702 to produce a signal at the output of NOR gate 701which is low during time intervals t₁ and t₂ and high during timeintervals t₃ and t₄ (see FIG. 7). This output of NOR gate 701 couples toan input of NAND gate 703 via conductor 705. Latch mode clock φ2 (FIG.11) couples to a second input of NAND gate 703 via conductor 512. Asshown in FIG. 7, latch mode clock φ2 is low during time interval t₃ andhigh during time intervals t₁, t₂ and t₄. Thus the coincidence of highlevels at the input of NAND gate 703 produces the output read-latchcommand φR at the output of NAND gate 703 which couples to conductor609. This signal is coupled to the input of inverter 704 whose outputcouples to conductor 609 which is the inverted output read-latch commandφR.

What is claimed is:
 1. An analog-to-digital converter comprising:(a)means for receiving an analog signal; (b) first comparator meansresponsive to said analog signal and including(i) means for providing afirst series of reference voltages ranging between a minimum voltage anda maximum voltage; (ii) a first plurality of differential comparisoncircuits, each having associated with it one of said first series ofreference voltages, each of said differential comparison circuitscomprisinga current splitting network having first and second terminalsfor providing first and second increments of current when said analogsignal exceeds said associated reference voltage; and means having athird terminal for providing a third increment of current when saidanalog signal is less than said associated reference voltage; (iii) aplurality of first output means each having an output terminal, each ofsaid first output means being coupled to said first terminal of anassociated differential comparison circuit and to said third terminal ofan adjacent differential comparison circuit, and a pair of second outputmeans each having an output terminal, one of said second output meansbeing coupled to said first terminal of the differential comparisoncircuit associated with said minimum voltage, and one of said secondoutput means being coupled to said third terminal of the differentialcomparison circuit associated with said maximum voltage, one of saidoutput means generating a first output signal at its output terminal inthe absence of any current increment input to such output means; and(iv) current summing means responsive to said second terminals foradding said second increments of current to form a reference current;(c) first encoder means responsive to said first output signal forproducing a first digital number; (d) subtractor means responsive tosaid analog signal and said reference current for producing anintermediate output signal; (e) second comparator means responsive tosaid intermediate output signal and including(i) means for providing asecond series of reference voltages; and (ii) comparison meansresponsive to said second series of reference voltages and saidintermediate output signal for generating a second output signal; and(f) second encoder means responsive to said second output signal forproducing a second digital number.
 2. The analog-to-digital converter asrecited in claim 1 wherein said first encoder means and said subtractormeans produce their respective first digital number and intermediateoutput signal substantially concurrently.
 3. The analog-to-digitalconverter as recited in claim 1 wherein said subtractor meanscomprisesoperational amplifier means having a non-inverting inputcoupled to said analog signal receiving means, an inverting input, andan output forming the output of said subtractor means; current mirrorcircuit means having a first terminal coupled to said reference currentand a second terminal coupled to said inverting input of saidoperational amplifier means, said current mirror circuit means producingat said second terminal a mirror current equal in magnitude to saidreference current; and impedance means coupled between said invertinginput and said output of said operational amplifier means.
 4. Theanalog-to-digital converter as recited in claim 1 wherein said first andsecond digital numbers are binary numbers.
 5. The analog-to-digitalconverter as recited in claim 1 wherein said first and second digitalnumbers are other than binary numbers.
 6. The analog-to-digitalconverter as recited in claim 1 wherein each of said differentialcomparison circuits in said first comparator further comprises:aconstant current source; a first mode select transistor having itsemitter coupled to said constant current source and its base coupled toa first clock signal; a second mode select transistor having its emittercoupled to said constant current source and its base coupled to a secondclock signal, said first and second clock signals being complementary;first and second input transistor having their emitters coupled to thecollector of said first mode select transistor and their bases coupled,respectively, to said analog signal and said associated referencevoltage; first and second cross-coupled latching transistors havingtheir emitters coupled to the collector of said second mode selecttransistor and having their collectors coupled, respectively, to thecollectors of said second and first input transistors; and wherein saidcurrent splitting network is coupled to the collector of said firstinput transistor and said means for providing a third increment ofcurrent is coupled to the collector of said second input transistor. 7.The analog-to-digital converter as recited in claim 1 wherein said meansfor providing a third increment of current in each of said differentialcomparison circuits comprises a second current splitting network havingsaid third terminal for providing said third increment of current, andhaving a fourth terminal for providing a fourth increment of currentwhen said analog signal is less than said associated reference voltage,and wherein said first comparator means further comprises additionalcurrent summing means responsive to said fourth terminals for addingsaid fourth increments of current to form an additional referencecurrent.
 8. The analog-to-digital converter as recited in claim 1wherein said first output means comprise dual emitter transistors andwherein said second output means comprise single emitter transistors. 9.The analog-to-digital converter as recited in claim 1 wherein saidcomparison means of said second comparator comprisesa second pluralityof differential comparison circuits, each having associated with it oneof said second series of reference voltages and including means forgenerating a fifth increment of current when said intermediate outputsignal exceeds said associated reference voltage and means forgenerating a sixth increment of current when said intermediate outputsignal is less than said associated reference voltage, and meansresponsive to said fifth and sixth increments of current for generatingsaid second output signal.
 10. The analog-to-digital converter asrecited in claim 9 wherein said means responsive to said fifth and sixthincrements of current comprises a plurality of dual emitter transistorseach having a first emitter responsive to one of said fifth incrementsof current and a second emitter responsive to one of said sixthincrements of current.
 11. The analog-to-digital converter as recited inclaim 6 wherein each of said differential comparison circuits furthercomprises first and second voltage level shifting circuits, said firstvoltage level shifting circuit being coupled between the collector ofsaid first input transistor and the base of said first cross-coupledtransistor, and said second voltage level shifting circuit being coupledbetween the collector of said second input transistor and the base ofsaid second cross-coupled transistor, whereby the switching speed ofsaid cross-coupled transistors is increased.
 12. The analog-to-digitalconverter as recited in claim 11 further comprising a power supply, andwherein each of said voltage level shifting circuits comprisea firsttransistor having a base coupled to the collector of said associatedinput transistor, a collector and an emitter; a resistor having firstand second terminals, said first terminal being coupled to the emitterof said first transistor; a second transistor having a base coupled tothe first terminal of said resistor, a collector coupled to thecollector of said first transistor and to said power supply, and anemitter coupled to the second terminal of said resistor; and a thirdtransistor having a collector coupled to the second terminal of saidresistor, a base coupled to the emitter of said second transistor, andan emitter coupled to the base of said associated cross-coupledtransistor.
 13. The analog-to-digital converter as recited in claim 1wherein said first and second digital numbers represent the mostsignificant bits and least significant bits, respectively, of theconverted analog signal.